Merge branch 'pw/3.0-default-initial-branch-to-main'
Declare that "git init" that is not otherwise configured uses
'main' as the initial branch, not 'master', starting Git 3.0.
* pw/3.0-default-initial-branch-to-main:
t0613: stop setting default initial branch
t9902: switch default branch name to main
t4013: switch default branch name to main
breaking-changes: switch default branch to main